H2-free Synthesis of Aromatic, Cyclic and Linear Oxygenates from CO2

Article abstract of DOI:10.1002/cssc.201902340

The synthesis of oxygenate products, including cyclic ketones and phenol, from carbon dioxide and water in the absence of gas-phase hydrogen has been demonstrated. The reaction takes place in subcritical conditions at 300 °C and pressure at room temperature of 25 barg. This is the first observation of the production of cyclic ketones by this route and represents a step towards the synthesis of valuable intermediates and products, including methanol, without relying on fossil sources or hydrogen, which carries a high carbon footprint in its production by conventional methods. Inspiration for these studies was taken directly from natural processes occurring in hydrothermal environments around ocean vents. Bulk iron and iron oxides were investigated to provide a benchmark for further studies, whereas reactions over alumina and zeolite-based catalysts were employed to demonstrate, for the first time, the ability to use catalyst properties such as acidity and pore size to direct the reaction towards specific products. Bulk iron and iron oxides produced methanol as the major product in concentrations of approximately 2–3 mmol L^?1. By limiting the hydrogen availability through increasing the initial CO₂/H₂O ratio the reaction could be directed to yield phenol. Alumina and zeolites were both observed to enhance the production of longer-chained species (up to C₈), likely owing to the role of acid sites in catalysing rapid oligomerisation reactions. Notably, zeolite-based catalysts promoted the formation of cyclic ketones. These proof-of-concept studies show the potential of this process to contribute to sustainable development through either targeting methanol production as part of a “methanol economy” or longer-chained species including phenol and cyclic ketones.

Full text of DOI:10.1002/cssc.201902340

Accepted Article
Title: H2-free synthesis of aromatic, cyclic and linear oxygenates from
CO2
Authors: Laura Quintana-Gómez, Amal Shehab, Ali Al-Shathr, William
Ingram, Mariia Konstantinova, Denis Cumming, and James
McGregor
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: ChemSusChem 10.1002/cssc.201902340
Link to VoR: http://dx.doi.org/10.1002/cssc.201902340
A Journal of
www.chemsuschem.org

ChemSusChem
10.1002/cssc.201902340
H2-free synthesis of aromatic, cyclic and linear oxygenates from CO2
a,b
a
a,
a
a
Laura Quintana Gomez, Amal K. Shehab, Ali Al-Shathr, William Ingram, Mariia Konstantinova,
a
a*
Denis Cumming and James McGregor
aUniversity of Sheffield, Department of Chemical and Biological Engineering, Mappin Street, Sheffield
S1 3JD, UK
bBioEcoUVa Bioeconomy Institute, Department of Chemical Engineering and Environmental
Technology, University of Valladolid, 47011 Valladolid, Spain
*
E-mail: james.mcgregor@sheffield.ac.uk
Abstract
The synthesis of oxygenate products, including cyclic ketones and phenol, from carbon dioxide and
water in the absence of gas-phase hydrogen has been demonstrated. The reaction takes place in sub-
critical conditions at 300 °C and pressure at room temperature of 25 barg. This is the first observation
of the production of cyclic ketones by this route, and represents a step towards the synthesis of
valuable intermediates and products, including methanol, without relying on fossil sources or
hydrogen - which carries a high carbon footprint in its production by conventional methods.
Inspiration for these studies is taken directly from natural processes occurring in hydrothermal
environments around ocean vents.
Bulk iron and iron oxides were investigated to provide a benchmark for further studies, while reactions
over alumina and zeolite-based catalysts have, for the first time, demonstrated the ability to use
catalyst properties such as acidity and pore size to direct the reaction towards specific products. Bulk
-
1
iron and iron oxides produce methanol as the major product in concentrations of ~2-3 mmol l . By
limiting the hydrogen availability through increasing the initial CO :H O ratio the reaction can be
2
2
directed to yield phenol. Alumina and zeolites are both observed to enhance the production of longer-
chained species (up to C8), likely due to the role of acid sites in catalysing rapid oligomerisation
reactions. Notably, zeolite-based catalysts promoted the formation of cyclic ketones.
These proof-of-concept studies show the potential of this process to contribute sustainable
development through either targeting methanol production, as part of ‘methanol economy’ or longer-
chained species including aromatics phenol and cyclic ketones.
Keywords
Carbon dioxide utilisation; High temperature water (HTW); Hydrothermal media; Phenol; Origin-of-
life
1
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ChemSusChem
10.1002/cssc.201902340
Graphical Abstract
1
0’s bar
Fe
H O H O
2
2
CO2
3
00 °C
2
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ChemSusChem
10.1002/cssc.201902340
1
. Introduction
Industrial chemicals and fuels are ubiquitous in modern society. Their production from
hydrocarbons, oxygenates, cyclic and aromatic compounds is essential for the global economy,
however an overwhelming proportion are produced from unsustainable sources. Sustainable
development is defined as, “development that meets the needs of the present without compromising
[
1]
the ability of future generations to meet their own needs” and hence new methods of chemical
-
1
production are essential in order to meet this goal. According to recent estimates, over 500 Mt yr of
fossil reserves are consumed by the chemical industry in the synthesis of necessary platform chemicals
[
2]
such as methanol, olefins and aromatics . Consequently, the chemical sector is the third largest
[
3]
industrial emitter of carbon dioxide (CO
2
) and a major contributor to anthropogenic climate change .
[
4]
This has stimulated investigations into carbon capture from power and chemical plants , e.g. as
[
5,6]
sodium bicarbonate (NaHCO
molecules presents an opportunity to utilise an abundant waste resource. Many methods of CO
conversion have been reported including hydrogenation, photochemical and electrochemical
3
)
. In this context, the direct conversion of CO to fuel and organic
2
2
[
7]
reduction , but low yields and/or high costs remain a major hindrance to adoption of these
technologies. Furthermore, the majority of methods of CO conversion require the use of hydrogen
gas, H , which is currently commercially obtained primarily from methane reforming . The use of H
is associated with high economic and environmental costs, as well as substantial safety concerns, in
particular flammability and leakage. Furthermore, 8.9 kg of by-product CO is produced per kg of H
from methane reforming, resulting in a large carbon footprint . Therefore, the direct conversion of
CO to value-added products in the absence of gaseous H is highly desirable. Perhaps surprisingly,
2
[
8]
2
2
2
2
[
9]
2
2
consideration of hypotheses of the origin-of-life on Earth may provide a source of inspiration as to
how this vision can be realised.
A widely discussed hypothesis is that the origin-of-life on Earth occurred around deep-sea
hydrothermal vents, where simple organic molecules can readily form. Temperatures and pressures
around these vents can reach 400 °C and 500 bar and CO can be present in concentrations up to
2
1[10–12]
~
500 mmol kg-
. French first proposed the synthesis of abiotic methane at hydrothermal vents by
[
13]
Fischer-Tropsch type catalytic reactions in the presence of iron-containing minerals . Generally,
hydrocarbons and other organic species in hydrothermal vents form from CO or other inorganic
carbon . Iron is the most abundant metal in the core of Earth and found throughout the ocean
2
[
14]
[
15]
floor , and its role in the conversion of CO to organic species has garnered a great deal of interest.
2
It is believed that iron-bearing minerals and sulphides behave as reductants for CO to organic species
2
at vent interfaces, with hydrogen provided through water splitting[16]. Therefore, the activity of bulk
iron and iron oxides in this reaction has been widely investigated. This concept has gained further
interest in the use of sub-critical water to abiotically convert CO
environments . Hydrothermal reactions take place in sealed pressure vessels in aqueous media at
conditions above 100 °C and 1 bar, at which point water exists in a sub-critical state . The properties
2
to organic species in laboratory
[
17]
[
18]
of sub-critical water differ greatly from that of water at room temperature, including high solubility of
organic substances and acceleration of acid and base-catalysed reactions[
19,20]
. A decrease in dielectric
[
19]
constant further makes it a good solvent for organic and hydrophobic substances .
The primary focus of CO reduction in hydrothermal media has been on the synthesis of short-chained
2
hydrocarbons such as methane, CH , and formic acid - a versatile platform molecule. The reduction of
4
aqueous carbon dioxide by zero-valent iron at ambient conditions in seawater solution was reported
21]
by Hardy and Gillham, who obtained hydrocarbons of chain length up to C
shorter reaction times of 40 days conversion to CH
at a pressure of 1 bar was achieved by Deng and co-workers . Increased yields of C
5
after 500 hours[ . At
4
and short-chain alkanes in the presence of Fe and
[
22]
-C
1 3
alkanes in the
3
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ChemSusChem
10.1002/cssc.201902340
presence of iron-containing olivine at 300 °C and 500 bar after 69 days were reported by Berndt et
[
17]
al. . At shorter reaction times of 20 hours, Guan et al. synthesised CH and trace methanol over zero-
4
[
23]
valent Fe . High yields of methanol are desirable as it is an excellent fuel for internal combustion
engines and a precursor for the production of formaldehyde, dimethyl ether, acetic acid and other
compounds[
24,25]
. At mildly hydrothermal conditions up to 200 °C, He et al. found that formic and
[
7]
acetic acids can be obtained with iron powder at reaction times of 2 hours . To illustrate the ability
of iron to generate H gas, Michiels et al. carried out a two-step hydrothermal reaction over zero-
valent Fe powder . First, H was produced, after which potassium carbonate, the CO source, was
2
[
26]
2
2
converted to formate, HCOO, with a 78% conversion and a selectivity in excess of 80%. Iron oxides
have also been found to be effective in this reaction. Chen et al. demonstrated that in the presence of
[
27]
magnetite, Fe
3
O , aromatic compounds are observed at 300 °C , while at higher temperatures of up
4
[
28]
to 350 °C organic acids are obtained . In these cases, the role of iron is to promote the reduction of
carbon dioxide. However, the reaction is most likely surface mediated, progressing through an iron
stabilised transition state. Therefore, iron can be considered as a catalyst, and may also play a role in
directing secondary reactions resulting in the formation ≥C products. It should be noted that other
2
relatively Earth-abundant metals with low redox potential, such as zinc, are also useful catalysts for
reduction[
29,30]
. Low reduction potential metals have been combined with other catalytic
CO
2
[
31,32]
[29,33,34]
substances, particularly nickel
and copper
, in order to enhance reaction kinetics and yields.
Additionally, reducing agents such as glycerol[ , biomass-derivatives and microalgae have been
35]
[36]
[37]
demonstrated as being effective in converting CO to HCOOH.
2
The studies reported above have predominantly focused on providing support for the hypothesis that
reactions around hydrothermal vents contributed to the development of species necessary for the
formation of life. However, a detailed understanding of these processes presents the possibility to
adapt and optimise these reactions for the synthesis of a wider range of products such as aromatics,
and longer-chained cyclic and linear oxygenates from CO
2
. The hydrothermal conversion of CO in the
2
absence of gas-phase H to species such as cyclic oxygenates has not previously been reported. Bulk
2
iron and iron oxides have been widely investigated in origin-of-life studies and thus can be used to
provide a benchmark for alternative catalysts and reaction conditions. Tailoring the product
distribution of this process through judicious selection of catalyst is herein presented for the first time.
The use of bifunctional catalysts containing both a metallic and an acidic zeolite component has
previously been proposed in other systems as a means of producing longer chain hydrocarbons in the
[
38]
gasoline range from gaseous CO
of larger species in hydrothermal media. In this work, the activity of iron supported on alumina,
Fe/Al , and two zeolite-supported catalysts, Fe/H-ZSM-5 and Fe/HY, as well as the bare zeolites, is
investigated. The primary aim is to develop a nature-inspired, H -free route to synthesise value-added
organic species from carbon dioxide and water with the aid of iron-containing catalysts.
2
and H
2
, and may therefore also present a route to the formation
2
O
3
2
2
. Experimental
A range of solid reductants have been utilised for reaction studies. The synthesis and
characterisation of these is described in Section 2.1. The reaction studies, and analysis of the reaction
products is described in Section 2.2.
2
.1 Solid reductants and catalysts
Two categories of solid reductants / catalysts have been utilised in this work: (i) bulk iron and iron
oxides; and (ii) supported iron catalysts. The bulk iron materials were Fe powder (≥ 99%, Sigma
4
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ChemSusChem
10.1002/cssc.201902340
Aldrich), Fe
2
O
3
(99% metal basis, Alfa Aesar) and Fe
3
O
4
(97% metal basis, Alfa Aesar); these were used
without further treatment or modification. The supported catalysts comprised an alumina supported
iron catalyst Fe/Al (35 % loading, Johnson Matthey Catalysts) and two zeolite supported catalysts
Fe/H-ZSM-5 and Fe/HY) synthesised in-house. The bare zeolites (H-ZSM-5 extrudate, SiO :Al mole
ratio = 1:38, ACS material; and H-zeolite Y powder, SiO :Al mole ratio = 5.1:1, Alfa Aesar) and a
commercial alumina (Puralox SBa200 (Sasol)) were also studied for comparison. The SiO :Al mole
2
O
3
(
2
2
O
3
2
2
O
3
2
2
O
3
ratios for the zeolites are supplied by the manufacturers. In the case of the extrudates this ratio refers
to the zeolite component; ZSM-5 is in the H-form in the supplied extrudates.
2
.1.1 Catalyst synthesis
Zeolite-supported iron catalysts were prepared via impregnation employing a method adapted from
[
39]
Lu et al. (2011) . Prior to impregnation, H-ZSM-5 extrudates were ground using a pestle and mortar.
To produce each catalyst, 5 g of each zeolite was mixed with 100 ml of 0.1 M aqueous solution of
Fe(NO
3
)
3 2
.9H O. The mixtures were then stirred for 12 h at 30 °C before being dried overnight at 120 °C.
Subsequently the catalysts were calcined in a furnace (MTI Corporation model KSL-1200 X) under static
-
1
air during which time the temperature was first increased to 120 °C at 10 °C min , and then held for
h before the temperature was further increased to 550 °C where it was then held for 5 h. The
synthesised catalysts were then cooled to room temperature, again under air.
1
2
.1.2 Catalyst characterisation
BET surface areas of the catalysts were determined using a 3 Flex Micromeritics Surface
Characterization instrument. Prior to analysis, catalysts were dried at 120 °C in a vacuum furnace.
Zeolite-based materials were also exposed to an in situ degasification whereby the temperature was
-
1
increased to 300 °C at 10 °C min and then held for 1 h.
The morphological characteristics of the catalysts, both fresh and post-reaction, were investigated by
SEM using a Jeol JSM-6010 LA Analytical Scanning Electron Microscopy. The accelerating voltage (V)
employed ranged from 15 kV to 20 kV and a working distance of 12 mm was used throughout.
Catalysts were pre-coated with gold for 10 s in an Agar Sputter Coater at 0.04 mbar and a current of
4
0 mA was applied.
In addition to SEM, the physical structure of the bulk iron and iron oxide catalysts after reaction was
also investigated by X-ray diffraction (XRD) in order to identify any changes in the bulk iron
composition. XRD patterns were recorded using a diffractometer (STOE STADI P) operated in
transmission mode at a voltage of 20 kV and a current of 5 mA. Data were collected at room
temperature in the 2θ range from 5° to 39.98° with a step size of 0.020° using Mo-Kα radiation.
Finally, any carbonaceous material (‘coke’) deposited during reaction over the bulk iron materials was
evaluated by temperature-programmed oxidation (TPO) employing a pulse chemisorption system
(
ChemiSorb 2720, Micromeritics), equipped with a Eurotherm 2416 temperature controller. The TPO
-
1
method consisted of fluxing in He for 30 min with a flow rate of 25 ml min to clean the catalyst
surface. Thereafter, the inert gas flow was switched 5 % O /He. After 20 min, the temperature was
increased to 950 °C with a heating rate of 10 °C min and held for 30 min.
2
-
1
2
.2 Reaction testing and product analysis
In all cases, the reactor (100 ml EZE-Seal Reactor, Parker Autoclave Engineers®, manufactured
from Hastelloy C) was loaded with 0.56 g of catalyst and 7 ml of distilled water. Gas chromatography-
mass spectrometry, GCMS, (Shimadzu QP2010SE, DB1-MS column, 60 m length, 0.25 mm i.d., 0.25 µm
film thickness) analysis indicated that the presence of organic matter in the water was negligible prior
to reaction. Subsequently, the reactor was twice flushed with CO
2
(purity 99.99 %, BOC) to eliminate
5
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ChemSusChem
10.1002/cssc.201902340
air. The system was then pressurised with CO
2
to ~25 barg (CO
2 2
:H O mole ratio = 0.26) and heated to
3
00 °C, resulting in an autogenous pressure increase. In studies investigating the influence of CO
2 2
:H O
ratio (Section 3.2.3) the reaction temperature was 350 °C and the step of flushing CO to eliminate air
2
was omitted. The reaction start-time was taken as the time at which the impeller (600 rpm) was turned
on. After the desired reaction period (4 h) the reaction was quenched by placing a water-ice bath
around the reactor.
Analysis of the gas phase was carried out at ~22 °C by mass spectrometry (HPR-20 QIC, Hiden
Analytical) (Table S1). The liquid and solid materials were then separated by vacuum filtration. The
solid residue was rinsed with water and dried overnight at ~110 °C. For the data presented in Section
3
.2, liquid-phase products were characterised by GCMS using a Shimadzu QP2012SE. 4-methyl-2-
pentanol (98 %, Sigma Aldrich) was used as an internal standard in a ratio of 1 µl to 1.5 ml of sample.
Quantification was conducted by calibration curves for the species identified. Full experimental details
for GCMS analysis are provided in the supplementary information.
CO
2
conversion (XCO2) was calculated according to Equation 2.1 and the selectivity (S ) according to
i
Equation 2.2:
X
CO2 = (nCO2
,
initial − nCO2
,
final / nCO2,initial ) × 100
(2.1)
(2.2)
S
i
i
=(n / nCO2,initial − nCO2,final ) × 100
where n
i
is the number of moles of a given product formed; nCO2,initial is the total number of moles of
CO present initially in both the gas and liquid-phase; and nCO2,final is the total number of moles of CO
after reaction in both the gas and liquid-phase.
2
2
3
. Results and Discussion
There are relatively few previous studies on the hydrothermal conversion of CO
2
in the absence
of an additional hydrogen source[
7,17,23,27,28,40]
. The majority of these have focussed on investigating the
role of such reactions in the production of molecules necessary for the evolution of life on Earth and
have hence principally considered the use of bulk iron powder and/or iron oxides as reductants or
catalysts. In the present work, the application of such reductants is therefore investigated in detail
initially (Section 3.2) in order to demonstrate the feasibility of this process in a conventional batch
reactor and to provide a benchmark against which to measure future process optimisation around the
use of alternative reductants and catalysts (Section 3.3). In advance of reaction studies, Section 3.1
presents details of the characterisation of the fresh reductants and catalysts.
3
.1 Catalyst characterisation
The calculated BET surface areas of the catalysts are shown in Table 1. The surface area of bulk Fe
powder is not shown as this is below the measurement limitations of the apparatus. As expected, the
bulk iron oxides show substantially lower surface areas than the zeolites and zeolite-supported
catalysts. The deposition of iron on the zeolite surface results in a decrease in surface area, both
through a loss of surface sites and through pore blockage.
Table 1. Measured BET surface areas of the reductants/catalysts employed. Error is ±2% of the
quoted values.
2
-1
Catalyst BET Surface area [m g ]
6
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ChemSusChem
10.1002/cssc.201902340
Fe
Fe
3
O
O
4
3
7
2
11
74
181
Fe/Al
2
O
3
γ-Al
2
O
3
Fe/H-ZSM-5 354
H-ZSM-5
Fe/HY
533
523
559
H-Zeolite Y
SEM was used to investigate the surface morphology of the catalysts prior to reaction. Representative
micrographs are shown in Figure 1.
a)
d)
g)
b)
e)
h)
c)
f)
i)
1
0 μm
5 μm
5
μm
ii
i
5
0 μm
10 μm
5 μm
5
μm
5 μm
5
μm
Figure 1: SEM micrographs of the reductants/catalysts employed in the hydrothermal reduction of
CO : (a) Fe powder; (b) Fe ; (c) Fe ; (d) Fe/Al ; (e) Al ; (f) H-ZSM-5; (g) Fe/ZSM-5; (h) HY; (i)
Fe/HY.
2
3
O
4
2
O
3
2
O
3
2
O
3
The differing morphology of the various catalysts employed is clearly demonstrated, e.g. with iron
powder showing both column- and plate-like grains (Figure 1(a)); iron oxides existing as clusters of
smaller particles; Al
2
O
3
and Fe/Al
2
O consisting of approximately spherical particles; and the zeolite
3
catalysts existing as clusters of varying sizes presenting inter-particles cavities alongside the intra-
particles channels present within their structure.
3
.2 Reactions over bulk iron powder and iron oxides
The hydrothermal conversion of CO over bulk iron powder (Section 3.2.1) and iron oxides
2
(Section 3.2.2) provides a benchmark for future studies, and a direct correlation to previous work.
7
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ChemSusChem
10.1002/cssc.201902340
Herein, particular attention is paid to relative quantities of carbon dioxide and water in the reaction
Section 3.2.3) with a view to altering the product distribution, specifically targeting longer chained
(
products and aromatic species alongside short-chained oxygenates such as methanol.
Characterisation of the reductants/catalysts after reaction is described in Section 3.2.4.
3
.2.1 Bulk iron powder
-
1
The hydrothermal conversion of CO
acetone (0.17 mmol l ), as shown in Figure 2. In addition, under the reaction conditions investigated,
longer-chained species were also produced in comparable amounts. Specifically, heptanal
2
over bulk iron powder yielded methanol (2.0 mmol l ) and
-
1
-
1
-1
(0.22 mmol l ) and 2-octanone (0.17 mmol l ), alongside trace amounts of cyclohexanone
-
1
-1
(0.06 mmol l ) and cycloheptanone (0.02 mmol l ). This high selectivity towards methanol is
advantageous in the development of processes within the ‘methanol economy’, however the
observation of significant concentrations of C and C products is also of particular interest. These
7
8
longer chained species can act as precursors for the synthesis of a number of products currently
derived from fossil resources, but are produced here directly from water and carbon dioxide. In
addition to liquid-phase products, the gas-phase species hydrogen, methane, acetaldehyde,
formaldehyde and ethene were identified over bulk Fe powder (Table S1). In total these represented
<
1% of the gas-phase species in the reactor post-reaction. In contrast, the quantities of methane and
ethene produced were 13 and 18 mol respectively. Hydrogen was the most abundant species
2.0 mmol) suggesting that hydrothermal conditions may facilitate water splitting as previously
(
[
16]
proposed . An alternative hypothesis is that H
CO and H
O, which is well-known to occur over iron-based materials[41,42]. In this case, reduction of
CO on the metal surface first leads to the formation of CO and iron oxides, which are active WGS
catalysts. Subsequently, in the presence of water, CO can be readily converted to CO and H gas. XRD
studies in the present work (Section 3.2.4) identified that some oxidation of bulk Fe indeed occurs.
However, it is noteworthy that H formation was also observed in reactions over iron-free catalysts,
e.g. γ-Al and zeolites (Table S1), which are not known as WGS catalysts. Therefore, at least portion
of H is proposed to arise from water splitting under hydrothermal conditions.
2
is formed via the water-gas shift (WGS) reaction of
2
2
2
2
2
2
O
3
2
It is also important to establish if any conversion occurs in the absence of a reductant or catalyst,
either induced thermally or promoted by the reactor walls, the latter being an oft under-identified
[43]
cause of chemical reaction . The reactor employed herein is constructed from Hastelloy C (58.6 %
Ni), and the conversion of CO into oxygenates and hydrocarbons has previously been demonstrated
2
[
31,32]
over nickel
~
. In the absence of a catalyst, the hydrothermal reaction of CO
2
at 300 °C, yielded
-
1
-1
1 mmol l of methanol and ~0.07 mmol l of acetone. Ethanol was also detected in trace amounts,
however, higher chained products were not observed. Therefore, while some activity is apparent
without adding an additional reductant/catalyst, the presence of iron powder substantially increases
8
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ChemSusChem
10.1002/cssc.201902340
the quantity of methanol produced and provides a new pathway to the synthesis of longer-chained
products.
1.2
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
Heptanal
-Octanone
Cyclopentanone
Cyclohexanone
Cycloheptanone
Methanol
Ethanol
Acetone
2
1.0
0.8
0.6
0.4
0.2
0.0
Fe powder Fe O
Fe O
Fe/Al O3 g-Al O
Fe powder Fe O4
Fe O3 Fe/Al O3 g-Al O3
3
4
2
3
2
2
3
3
2
2
2
Catalyst
Catalyst
Figure 2: Distribution of liquid-phase products over iron, iron oxide and alumina based
reductants/catalysts. Left hand plot shows low carbon number products; right hand plot shows
higher carbon number products. Reaction conditions: T = 300˚C, CO :H O mole ratio = 0.26, t = 4 h,
.56 g catalyst.
2
2
0
The reactivity observed over bulk iron powder therefore confirms the ability of iron to promote the
formation of oxygenates from CO under hydrothermal conditions. This contributes to the body of
2
evidence that such reactions may provide a role in the formation of molecules necessary for the
evolution of life on Earth, and also provides a valuable baseline from which to compare the behaviour
of other reductants and catalysts in the on-going drive to develop sustainable industrial routes to
hydrocarbons and oxygenates taking inspiration from such natural reactions.
3
.2.2 Bulk iron oxides
The active surface phase and oxidation state of iron under hydrothermal reaction conditions is the
subject of debate with, for example, the formation iron oxyhydroxide having been previously
characterised in both natural environments around hydrothermal vents and in laboratory studies
under aqueous conditions at elevated temperature[
44–47]
. As iron has been proposed to act directly as
a reductant and oxygen sink in this reaction it might be expected that more oxidised forms of iron are
less favourable in promoting CO hydrogenation and polymerisation. Fe(II/III) and Fe(III) oxides – Fe
and Fe respectively – have therefore been investigated in place of bulk iron powder (Section 3.2.1).
2
3
O
4
2
O
3
As bulk materials with no, or low, porosity, Fe and Fe have intrinsically low surface areas of 7
3
O
4
2
O
3
2
-1
and 11 m g respectively (Table 1). As with Fe powder however, they demonstrate clear and
measurable activity towards CO conversion. The CO conversions achieved over Fe and Fe were
4% and 11% respectively, which compares with 7% over bulk Fe. The higher conversion over Fe is
achieved despite having a lower surface area than Fe , while conversion over Fe powder is 64% of
that achieved over Fe despite the former having a surface area below the measurable limit. It is
2
2
3
O
4
2
O
3
1
3
O
4
2
O
3
2
O
3
therefore apparent that conversion does not exhibit a simple direct correlation with surface area and
that among the oxide materials, the more reduced species has higher activity, as predicted. All species
are however active for the hydrothermal conversion of CO
2
.
Figure 2 shows the product distribution observed in the liquid phase over the bulk oxides. The major
-
1
product formed in both cases was methanol with a concentration ~2-3 mmol l , similar to that over
9
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ChemSusChem
10.1002/cssc.201902340
bulk iron powder, with Fe
2
O
3
yielding the highest concentration. Acetone production is also observed.
The methanol:acetone ratios over Fe, Fe
production of the C ketone is therefore more favourable, with respect to methanol, over the oxides.
This trend follows the increase in CO conversion over the three materials, therefore acetone may be
formed through successive CO addition to methanol.
3
O
4
and Fe
2
O are 12:1, 6:1 and 9:1 respectively. The
3
3
2
2
Analysis of the reactor headspace after reaction indicated the presence of methane, acetaldehyde,
formaldehyde and ethene, alongside CO (Table S1). These products represent 12% and 9% of the gas-
phase species over Fe
Fe; this difference being a consequence predominately of elevated of CO formation over the oxides
where the selectivity to CO was 79 % and 74% respectively. Fe and Fe also exhibited higher
hydrocarbon production, yielding approximately 18 µmol CH and in excess of 30 µmol C (Table
S1). The formation of CO from CO over iron materials is well known, in particular via the reverse
water-gas shift reaction (RWGS). Indeed, RWGS has been proposed as key step in the synthesis of
3
O
4
and Fe
2
O
3
promoted reactions respectively. This compares to <1% over bulk
3
O
4
2
O
3
4
H
2 4
2
higher hydrocarbons from CO
wise the hydrogenation of CO
2
through a modified Fischer-Tropsch reaction (Scheme 1) involving step-
and CO with gas-phase H
2
over supported iron catalysts[
48–51]
:
2
1
2
. CO
2
+ H
2
⇌ CO +H
2
O
H573K = 38 kJ mol-1
. CO + 2H
2
→ (CH
2
)
n
+ H
2
O
H573K = -166 kJ mol-1
Scheme 1
Methanol, ethanol and longer chained alcohols and other oxygenates can also be produced via this
route where step 2 in Scheme 1 becomes:
2
CO + 4H
2
3 2 2
→ CH CH OH + H O
Scheme 2
with >C
2
alcohols formed via polymerisation of ethanol[52–55]
.
The formation of CO via the RWGS reaction may therefore provide an alternative route to the
formation of C2+ oxygenates under hydrothermal conditions. In analogous gas-phase reactions over
solid catalysts it is known that the synthesis of C2+ species from CO
formation of CO as a key intermediate via the RWGS reaction . However, in the present work the
2
/H mixtures first involves in situ
2
[
56]
relative absence of CO over bulk Fe powder suggests that it may not be a pre-requisite and that higher
products may be formed directly from CO . In order to confirm the mechanistic role of CO in long-
2
chain oxygenate formation, future studies should consider spiking the reaction with increasing
amounts of CO.
3
.2.3 Synthesis of aromatics over bulk oxides
Aromatic compounds have higher C:H ratios than linear or other cyclic species, therefore it may be
expected that a higher reaction temperature and increasing the CO :H O ratio in the reactor could
2
2
direct the reaction towards the formation of such products. The synthesis of phenol under
hydrothermal conditions has previously been observed by Tian and co-workers, with yields of up to
-
1
0
(
.8 mol.% (0.4 mmol l ) observed using sodium bicarbonate as the CO
2
source and 1.2 mol.%
. Figure 3 compares the production of phenol to other
) species, both linear and cyclic, in the present work over Fe powder. It
is clear from these data that upon increasing CO :H O from 0.25 to 0.35 and then further to 0.60 the
-
1
[57,58]
1.1 mmol l ) using gas-phase CO
2
directly
high carbon-number (C to C
5
8
2
2
-
1
production of phenol is significantly enhanced, with a yield of 4.5 mmol l at the highest ratio; this is
significantly higher than previously observed yields. At the same time, the reaction increasingly
favours longer-chained species at higher CO
coupled with an increase in the production of 2-octanone. The mole ratio of 2-octanone to heptanal
increases from 0.5:1 at a CO :H O ratio of 2.5 to 1.6:1 at a CO :H O ratio of 0.6.
2
:H O ratios with a decrease in the production of heptanal
2
2
2
2
2
1
0
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These data strongly support the hypothesis, tested here for the first time, that the product distribution
can be tailored to produce not only longer hydrocarbon chains but also desirable aromatic species
which are particularly valuable given the current reliance on crude oil for their production and the
current challenges in producing aromatic species from simple molecules such as CO
2
via conventional
5
4
3
2
1
0
2.0
.8
1.6
.4
1.2
.0
0.8
.6
0.4
.2
0.0
Heptanal
-Octanone
Phenol
Cyclopentanone
Cyclohexanone
Cycloheptanone
Cyclooctanone
1
2
1
1
0
0
0.25
0.35
0.6
0.25
0.35
CO :H O Ratio (mol)
0.6
CO :H O Ratio (mol)
2
2
2
2
routes.
Figure 3: Distribution of liquid-phase products over iron powder with varying CO
2
:H O ratios. Left
2
hand plot shows linear and aromatic products; right hand plot shows (non-aromatic) cyclic products.
Reaction conditions: T = 350˚C, t = 4 h, 0.56 g catalyst.
3
.2.4 Evolution of materials during reaction
SEM was used in order to determine if any significant morphological changes were induced in the
catalysts by the processing conditions employed (Figure 4 (a)-(c)). Comparing Figures 1 and 4, there
are only very limited morphological changes evident after reaction. Most notable is the increase in
surface roughness, in part ascribable to the presence of surface carbonaceous deposits, vide infra.
a)
d)
g)
b)
e)
h)
c)
f)
i)
1
0 μm
5
μm
5 μm
1
0 μm
5
0 μm
5 μm
5
μm
5 μm
5 μm
1
1
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Figure 4: Post-reaction SEM micrographs of the reductants/catalysts employed in the hydrothermal
reduction of CO : (a) Fe powder; (b) Fe ; (c) Fe ; (d) Fe/Al ; (e) Al ; (f) H-ZSM-5; (g)
2
3
O
4
2
O
3
2
O
3
2
O
3
Fe/ZSM-5; (h) HY; (i) Fe/HY. The circled area in (a) highlights an area identified as containing
carbonaceous deposits.
XRD was employed to investigate changes in the iron phases present in the bulk iron and iron oxides
after reaction. As shown in Figure 5, the peaks observed in the post-reaction Fe
3
O
4
and Fe
2
O XRD
3
patterns correspond to the original Fe and Fe crystalline phases respectively. Spent Fe powder
3
O
4
2
O
3
exhibited the three characteristic peaks of metallic α-Fe at 2θ = 44.68°, 65.03° and 82.21°. In addition,
a low intensity peak was observed at 35.36° (indicated by an arrow in Figure 5), which can be assigned
[
59]
to Fe
3
O
4
. Thus, these findings suggest that a slight oxidation of Fe powder catalyst may have
occurred during the hydrothermal reaction. Although no significant changes were observed in the bulk
phase of the oxide catalysts, and therefore they do not undergo significant bulk oxidation,
transformation in the composition of the catalyst surface, such as changes in the oxidation state of
the Fe, may have occurred.
Fe O
2
3
Fe O
3
4
Fe powder
20
40
60
80
2q (degree)
Figure 5: XRD patterns for spent Fe powder, Fe and Fe
3
O
4
2
O
3
catalysts. The arrow indicates a low
intensity peak at 35.36° indicative of Fe
3 4
O .
The nature and quantity of carbon deposited on the catalysts was investigated through TPO. TPO
profiles for iron powder, Fe and Fe after 4 h of reaction are shown in Figure 6. There are clear
3
O
4
2
O
3
differences in both the total amount of carbon deposited and the oxidation temperature – and hence
structure – of the coke. Iron powder, despite having the lowest surface area, has both the greatest
quantity of, and most structured, coke deposits. The latter is revealed by the high oxidation
temperature, with the major peak centred at ~750 °C, alongside a shoulder at ~600 °C. This
temperature is typical of graphitic structures[ . These data are also consistent with the carbon
balance over the catalysts which show an imbalance between the quantity of carbon introduced as
60]
CO initially and that present in the liquid and gas-phase after reaction. The difference between the
2
1
2
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initial and final carbon quantities was 6.5% over Fe and 2.1% and 2.0% over Fe
3
O
4
and Fe
2
O
3
respectively. This greater imbalance over Fe is in line with the TPO data showing more retained carbon
on the Fe surface as compared to the oxides.
0.10
0.08
0.06
0.04
0.02
0.00
Fe powder
3
Fe O4
Fe O3
2
200
400
600
800
Temperature (°C)
Figure 6: TPO profile of spent Fe powder and Fe oxide catalysts. Reaction conditions: T = 300 °C,
CO
2
:H
2
O mole ratio = 0.26, t = 4 h, 0.56 g catalyst. Mass of spent catalyst employed in TPO
analysis = 35-45 mg.
Comparing the two bulk oxides, the TPO profile of Fe
3
O
4
exhibits a single symmetric peak at ~260 °C,
typical of amorphous carbon deposits . In contrast, the Fe profile exhibits three broad peaks at
50 °C, 550 °C and 850 °C corresponding to different surface carbon species. The lower temperature
peaks are significantly larger in area than that at 850 °C, indicating that overall the carbon deposits
formed over Fe are dominated by less structured materials than those formed over iron powder.
[
60]
O
2 3
3
2
O
3
3
.2.5 Summary of studies over bulk iron / iron oxides
Bulk iron was confirmed as possessing the ability to promote the hydrothermal conversion of CO
the absence of gas-phase H , producing methanol as the major product. Additionally, the product
2
in
2
distribution can be tailored by varying reaction conditions, with enhanced CO
valuable aromatic product phenol.
2
:H O ratios yielding the
2
Despite being in a higher oxidation state than bulk iron and hence having reduced capacity as a
reductant and oxygen sink, it is also apparent that Fe and Fe have activity towards CO
conversion under hydrothermal conditions. Comparing the oxides, the more oxidised Fe does show
2
O
3
3
O
4
2
2
O
3
the lowest conversion which may in part be related to its higher oxidation state. The surface-mediated
reaction is however clearly dependent on a number of underpinning factors and a future detailed
study, ideally employing advanced operando methods, would be necessary to deconvolve these
influences. That more oxidised forms of iron are not inactive is a critical observation and shows that
the system will not be deactivated directly through the surface oxidation of iron in high temperature
water.
1
3
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3
.3 Reactions over supported catalysts
While iron and iron compounds such as iron oxides are present naturally around deep sea
hydrothermal vents, judicious selection and design of catalysts presents an opportunity to improve
the efficiency of this process and to target a broader range of products. In order to exploit this
reactivity as a means of synthesising alcohols and other oxygenate products from CO it is necessary
2
to optimise the catalyst and reductants used. Industrial heterogeneous catalysts are typically prepared
with the active species dispersed onto a support resulting in greater availability of the active metal
surface for reaction. Herein, an alumina supported iron catalyst (Section 3.3.1) is tested to compare
to the bulk oxides, while zeolite-supported iron catalysts have been investigated as potential
bifunctional materials to alter the product distribution (Section 3.3.2). The use of bifunctional catalysts
containing both an acidic zeolite component has previously been proposed as a means of upgrading
the products from Fischer-Tropsch synthesis from CO
range . This employs a sodium-promoted iron catalyst for the initial CO conversion via reverse
2
to longer chain hydrocarbons in the gasoline
[
38]
2
water-gas shift and Fischer-Tropsch reactions, while the zeolite component catalyses oligomerisation,
isomerisation and aromatisation reactions. Therefore, zeolite supported catalysts may produce longer
chained, or cyclic, species. Bare alumina and the zeolite supports have also been tested for
comparison.
3
.3.1 Alumina and alumina-supported iron
Figure 2 shows the extent of production of both short and long-chained oxygenate products over
Fe/Al and Al alongside the bulk iron and iron oxides. Fe/Al shows similar yields of products
2
O
3
2
O
3
O
2 3
to the bulk materials overall, but with somewhat enhanced production of longer-chained products.
The methanol:acetone ratio is 3.7:1, lower than that over the bulk materials, while greater quantities
of heptanal and 2-octanone are also observed. This is achieved despite a CO
2
conversion of only 1.1%.
, which yields 1.1 mol l 2-octanone; over six times
that produced in the presence of iron powder despite a CO conversion of only 7.7%. Acid sites on
g-Al therefore favour the formation of long-chained products and the presence of iron active sites
is not a pre-requisite. CO , H , H O, acetaldehyde and formaldehyde are all observed as products in
-
1
This trend is even more pronounced over g-Al
2
O
3
2
2
O
3
2
2
2
the gas-phase (Table S1). As with the bulk materials, SEM data (Figure 4 (d)-(e)) show limited
differences between the pre- and post-reaction samples, with some agglomeration observed to have
taken place for Fe/Al
2
O
3
.
3
.3.2 Zeolite and zeolite-supported iron
The use of zeolite-supported iron catalysts allows the benefits of a bifunctional catalyst to be probed,
where the zeolite acid sites may catalyse secondary reactions of the primary alcohol products. Figure
7
quantitatively compares the amount of both short and long-chained oxygenates produced over the
zeolite and iron-zeolite catalysts. As with bulk iron powder and iron oxides, methanol is the organic
-
1
product produced in the greatest quantity, with all four zeolite-based materials yielding ~3 mmol l .
The production of acetone shows a clear dependence on the nature of the zeolite used, and hence on
-
1
zeolite acidity. Both Fe/H-ZSM-5 and H-ZSM-5 produced <0.25 mmol l whereas over the less acidic
-
1
-1
Fe/HY and HY, >0.5 mmol l and >1 mmol l were produced respectively. Similar gas-phase species
were evolved as over iron and alumina materials (Table S1), while SEM data again show only limited
changes after reaction such as the presence of some carbonaceous deposits.
Comparing the production of longer-chained products over zeolite-based catalysts to that over the
bulk iron powder and bulk iron oxides (Figure 2), a clear difference is the presence of cyclic ketones
with five to seven carbons in significant concentrations, e.g. eighteen-times more cyclopentanol is
produced over HY than over Fe
lower concentrations, e.g. H-ZSM-5 yielded 0.07 mmol l of heptanal. It is also notable that the
2
O . In contrast, linear species in the same size range were present in
3
-
1
1
4
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10.1002/cssc.201902340
unmodified zeolites have comparable performance to the iron-impregnated zeolites, and indeed
unmodified HY exhibits the greatest yields of cyclopentanone and cyclohexanone among all materials
investigated. This supports the conclusion of studies on g-Al
2
O
3
(Section 3.3.1) that iron is not required
for the hydrothermal conversion of CO
of effecting this transformation.
2
and that active sites on zeolites and alumina are also capable
The formation of cyclic compounds most likely occurs via cyclisation reactions of linear species and
can hence be considered as a secondary process. Zeolites are well known cyclisation catalysts, e.g.
[61,62]
cyclisation is a key step in the methanol-to-hydrocarbons reaction over H-ZSM-5
; the mechanism
of which is widely discussed. Elsewhere, the cyclisation of surface adsorbed carbonaceous species has
[
63]
been identified as a key step in the formation of coke deposits on zeolites . The tendency to form
larger carbon-number rings in HY cf. H-ZSM-5 may be ascribed to the larger pore size of the former.
Detailed theoretical studies of hydrocarbon oligomerisation-cyclisation reactions over zeolite catalysts
[
64–66]
have previously provided insights into the reaction mechanism
; with hydrocarbons proposed to
adsorb at framework oxygen species in the zeolite structure. Subsequent dehydrogenation of the
cyclic products, through a hydride transfer mechanism, can yield aromatic products such as those
observed in this work under alternative reaction conditions (Section 3.2.3). This latter reaction can be
[
67]
enhanced in the presence of CO
2
which can act as a hydrogen-acceptor .
The product concentrations in the liquid-phase after reaction compare favourably with alternative
liquid-phase processes. The electrochemical conversion of CO to methanol provides an alternative
route to this key oxygenate intermediate. Reported methanol concentrations achieved
2
-
1 [25]
electrochemically range from 0.001 to 6.5 mmol l
experimental conditions, produce a final liquid-phase product that compares well with these despite
being at an early proof-of-concept stage. This highlights the potential role for hydrothermal CO
. The results herein, albeit under very different
2
conversion in supporting the methanol economy, in addition to its ability to yield higher hydrocarbons
and aromatics.
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
1.0
0.8
Methanol
Ethanol
Acetone
Butanal
-Butanone
Heptanal
Cyclopentanone
Cyclohexanone
Cycloheptanone
2
0.6
0.4
0.2
0.0
Fe/ZSM-5
ZSM-5
Fe/HY
HY
Fe/ZSM-5
ZSM-5
Fe/HY
HY
Catalyst
Catalyst
Figure 7: Distribution of liquid-phase products over zeolite and zeolite-supported iron catalysts. Left
hand plot shows low carbon number products; right hand plot shows higher carbon number
products. Reaction conditions: T = 300˚C, CO
2
:H O mole ratio = 0.26, t = 4 h, 0.56 g catalyst.
2
3
.3.3 Summary of studies over supported catalysts
Alumina and zeolites are both observed to enhance the production of longer-chained and cyclic
species respectively. This suggests that acid sites present on both of these materials play a key role,
1
5
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ChemSusChem
10.1002/cssc.201902340
e.g. through catalysing oligomerisation reactions, and that metallic sites are not a pre-requisite for
catalytic activity. Supporting the conclusion that acid sites play a critical role, complementary studies
on carbon-based catalysts lacking acidic functionalities do not show the production of longer-chained
species [ . Notably, zeolite-based catalysts promoted the formation of cyclic ketones. To the best
knowledge of the authors this represents the first report of such products arising from hydrothermal
68]
CO conversion. These results therefore confirm the possibility to alter, and hence tailor, product
2
distribution through the development of robust structure-performance relationships. To this end,
Section 3.4 presents initial hypotheses as to the reaction mechanism.
3
.4 Reaction mechanism
The observation of longer chained products (up to C
8
) in the studies reported herein is highly
significant. The production of aromatic and cyclic species is also of great value, however these most
likely form via secondary reactions of linear species; for instance, cyclisation over acid sites present
on zeolite supports (Section 3.3.2). It is therefore desirable to understand the reaction mechanism
leading to the formation of these longer chained species on iron surfaces in order to ultimately
develop structure-performance relationships and hence inform future catalyst design.
It is possible that the reaction proceeds via either formate (HCOO) (Figure 8(a)) or carboxylate (COOH)
as an intermediate (Figure 8(b)). It has previously been reported that the adsorption of HCOO on iron
surfaces is more stable than that of carboxylate (COOH)[
69,70]
. Thus, it may be expected that the first
step in the mechanism of formation of heptanal and 2-octanone is the formation of formate. However,
it has been observed (e.g. Figure 2) that liquid species formed during the hydrothermal conversion of
CO
2
in the present work maintained the C=O double bond. This is consistent with the carboxylate
mechanism as the C=O bond is preserved through all steps.
a)
b)
Figure 8: Reaction mechanism for the production of heptanal and 2-octanone via (a) a
formate intermediate and (b) a carboxylate intermediate.
1
6
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A third possibility that could explain the observation of heptanal and 2-octanone is that the
mechanism may proceed via aldol condensations reactions (Figures 9(a) and 9(b) respectively). The
initial reactants for this process are short-chained oxygenates such as acetaldehyde, formaldehyde
and acetone – all of which are identified as products in the present work. The identification of these
crucial intermediates in the reaction from CO to longer-chained oxygenates provides some support
2
for this hypothesis. It is however notable that branched-chained species are common products of aldol
condensations, and that in the present work such branched species were not observed. In the
[
71]
microporous zeolite catalysts this may be ascribed to product shape-selectivity ; this however is
unlikely to be applicable to, e.g. bulk Fe powder where heptanal and 2-octanone are also observed.
a)
b)
Figure 9: Reaction mechanism for the production of (a) heptanal and (b) 2-octanone via
aldol condensation reactions.
In order to provide further insights into the reaction mechanism the influence of adding oxygenates
(methanol and 2-propanol) to the original reaction mixture, and also of replacing water entirely with
methanol and 2-propanol has been investigated. These data show that the presence of alcohols
enhances the production of long-chained species such as heptanal and 2-octanol (Figure S1), and also
results in the formation of additional long-chained species (Figure S2). However, at least a significant
fraction of this conversion can be ascribed to the direct reaction of the alcohols rather than their
reaction with CO
methanol to higher oxygenates may occur via the condensation of intermediates such as
formaldehyde and formyl rather than via the decomposition of methanol into CO and H followed by
a higher-alcohol synthesis (HAS) route. Therefore, in this case, CO may promote the formation of the
2
, as shown by studies under a He atmosphere (Figure S3). The conversion of
2
2
reaction intermediates, and thus, via subsequent condensation, enhance the production of higher
oxygenates. The influence of other oxygenates such as sugars, alcohols, aldehydes and acids on the
reduction of CO
the production of formic acid . In those studies, the oxygenates were concluded to act as a
hydrogen-donor for CO reduction. The data presented herein therefore indicate that a number of
competing parallel reaction mechanisms may operate. It is not yet clear which of these mechanisms
2
(from NaHCO
3
) has also previously been investigated by other researchers targeting
[
72]
2
1
7
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ChemSusChem
10.1002/cssc.201902340
dominates. This is a key area for future investigations and will facilitate the development of new
reductants and catalysts to optimise this process.
4
. Conclusions and Outlook
This work has detailed proof-of-concept studies which have confirmed the ability of iron and
iron-based materials to promote the hydrothermal conversion of carbon dioxide to oxygenate species
in the absence of gas-phase hydrogen. Direct inspiration for this work is drawn from reactions that
take place in the natural environment around hydrothermal vents. Crucially, the ability to direct the
product distribution of this reaction through the choice of catalysts and reaction conditions has been
demonstrated for the first time, with significant scope for future process optimisation. For instance,
nickel and cobalt are also known to activate CO while the acidity and/or pore size of support materials
2
can direct for specific carbon-chain length or ring size. A feature of this process is the ability to target
either methanol production, which underpins the so-called ‘methanol economy’ or longer-chained
species including aromatics (phenol) and cyclic ketones, the production of which via this route had not
previously been reported.
Acknowledgements
The authors thank Johnson Matthey Catalysts for provision of the Fe/Al
2
O catalyst. EPSRC is
3
acknowledged for support via grant EP/R026815/1 and via EP/K001329/1, for SEM data, and for
funding CO2Chem Grand Challenge Network (EP/K007947/1 and EP/H035702/1) which proved
support for WI. AA-S acknowledges the Iraqi Ministry of Higher Education and Scientific Research and
the Chemical Engineering Department, University of Technology, Baghdad, Iraq.
References
[
1]
World Commission on Environment and Development., Our Common Future, Oxford
University Press, 1987.
[
[
2]
3]
P. G. Levi, J. M. Cullen, Environ. Sci. Technol. 2018, 52, 1725–1734.
M. van der Van, Y. Kobayashi, R. Diercks, Technology Roadmap: Energy and GHG Reductions
in the Chemical Industry via Catalytic Processes, Paris, 2013.
[
[
[
[
[
[
[
4]
5]
6]
7]
8]
9]
N. A. Razali, M. Conte, J. McGregor, Catal. Letters 2019, 149, 1403–1414.
E. S. Sanz-Pérez, C. R. Murdock, S. A. Didas, C. W. Jones, Chem. Rev. 2016, 116, 11840–11876.
D. W. Keith, G. Holmes, D. St. Angelo, K. Heidel, Joule 2018, 2, 1–22.
C. He, G. Tian, Z. Liu, S. Feng, Org. Lett. 2010, 12, 649–651.
B. Michalkiewicz, Z. C. Koren, J. Porous Mater. 2015, 22, 635–646.
G. Centi, E. A. Quadrelli, S. Perathoner, Energy Environ. Sci. 2013, 6, 1711–1731.
10] N. J. Pester, E. P. Reeves, M. E. Rough, K. Ding, J. S. Seewald, W. E. Seyfried, Geochim.
Cosmochim. Acta 2012, 90, 303–322.
[
[
11] D. M. Kerrick, M. A. McKibben, T. M. Seward, K. Caldeira, Chem. Geol. 1995, 121, 285–293.
12] L. Li, X. Zhang, Z. Luan, Z. Du, S. Xi, B. Wang, L. Cao, C. Lian, J. Yan, Geochemistry, Geophys.
Geosystems 2018, 19, 1809–1823.
[
13] B. M. French, (1) Synthesis and Stability of Siderite, FeCO3, (2) Progressive Contact
Metamorphosis of the Biwabik Iron Formation on the Mesabi Range, John Hopkins University,
Baltimore, Maryland, 1964.
[
[
[
14] T. M. McCollom, J. S. Seewald, Chem. Rev. 2007, 107, 382–401.
15] P. A. Frey, G. H. Reed, ACS Chem. Biol. 2012, 7, 1477–1481.
16] R. He, B. Hu, H. Zhong, F. Jin, J. Fan, Y. H. Hu, Z. Jing, Chem. Commun. 2019, 55, 1056–1059.
1
8
This article is protected by copyright. All rights reserved.

ChemSusChem
10.1002/cssc.201902340
[
[
[
[
[
[
[
[
17] M. E. Berndt, D. E. Allen, W. E. Seyfried, Geology 1996, 24, 351.
18] A. Rabenau, Angew. Chemie Int. Ed. English 1985, 24, 1026–1040.
19] S. S. Toor, L. Rosendahl, A. Rudolf, Energy 2011, 36, 2328–2342.
20] N. G. Holm, E. Andersson, Astrobiology 2005, 5, 444–460.
21] L. I. Hardy, R. W. Gillham, Environ. Sci. Technol. 2002, 30, 57–65.
22] B. Deng, T. J. Campbell, D. R. Burris, Environ. Sci. Technol. 1997, 31, 1185–1190.
23] G. Guan, T. Kida, T. Ma, K. Kimura, E. Abe, A. Yoshida, Green Chem. 2003, 5, 630–634.
24] A. Goeppert, M. Czaun, J. P. Jones, G. K. Surya Prakash, G. A. Olah, Chem. Soc. Rev. 2014, 43,
7
995–8048.
[
[
25] J. Albo, M. Alvarez-Guerra, P. Castaño, A. Irabien, Green Chem. 2015, 17, 2304–2324.
26] K. Michiels, B. Peeraer, W. Van Dun, J. Spooren, V. Meynen, Faraday Discuss. 2015, 183, 177–
1
95.
[
[
[
[
27] Q. Chen, Y. Qian, Chem. Commun. 2001, 0, 1402–1403.
28] Q. W. Chen, D. W. Bahnemann, J. Am. Chem. Soc. 2000, 122, 970–971.
29] Z. Huo, M. Hu, X. Zeng, J. Yun, F. Jin, Catal. Today 2012, 194, 25–29.
30] D. Roman-Gonzalez, A. Moro, F. Burgoa, E. Pérez, A. Nieto-Márquez, Á. Martín, M. D.
Bermejo, J. Supercrit. Fluids 2018, 140, 320–328.
[
31] H. Takahashi, L. H. Liu, Y. Yashiro, K. Ioku, G. Bignall, N. Yamasaki, T. Kori, J. Mater. Sci. 2006,
4
1, 1585–1589.
[
[
[
[
[
32] B. Wu, Y. Gao, F. Jin, J. Cao, Y. Du, Y. Zhang, Catal. Today 2009, 148, 405–410.
33] H. Zhong, F. Jin, B. Wu, H. Chen, G. Yao, AIP Conf. Proc. 2010, 1251, 213–216.
34] L. Y. Wang, G. D. Yao, Z. Z. Jing, F. M. Jin, Adv. Mater. Res. 2014, 1073–1076, 39–42.
35] J. Su, L. Yang, X. Yang, M. Lu, B. Luo, H. Lin, ACS Sustain. Chem. Eng. 2015, 3, 195–203.
36] M. Andérez-Fernández, E. Pérez, A. Martín, M. D. Bermejo, J. Supercrit. Fluids 2018, 133, 658–
6
64.
37] Y. Yang, H. Zhong, R. He, X. Wang, J. Cheng, R. He, G. Yao, F. Jin, Green Chem. 2019, 21, 1247–
252.
[
1
[
[
[
[
38] J. Wei, Q. Ge, R. Yao, Z. Wen, C. Fang, L. Guo, H. Xu, J. Sun, Nat. Commun. 2017, 8, 15174.
39] J. Lu, Y. Liu, N. Li, J. Nat. Gas Chem. 2011, 20, 423–427.
40] T. M. McCollom, J. S. Seewald, Geochim. Cosmochim. Acta 2001, 65, 3769–3778.
41] J. Dufour, C. Martos, A. Ruiz, in Hydrog. Prod. Prospect. Process., Nova Science Publishers,
Inc., 2012, pp. 175–200.
[
[
[
42] M. Zhu, I. E. Wachs, ACS Catal. 2016, 6, 722–732.
43] C. H. Collett, J. McGregor, Catal. Sci. Technol. 2016, 6, 363–378.
44] B. M. Toner, C. M. Santelli, M. A. Marcus, R. Wirth, C. S. Chan, T. McCollom, W. Bach, K. J.
Edwards, Geochim. Cosmochim. Acta 2009, 73, 388–403.
[
[
[
[
[
45] O. Rouxel, B. Toner, Y. Germain, B. Glazer, Geochim. Cosmochim. Acta 2018, 220, 449–482.
46] R. T. Wilkin, M. S. McNeil, Chemosphere 2003, 53, 715–725.
47] J. L. Jambor, J. E. Dutrizac, Chem. Rev. 2003, 98, 2549–2586.
48] S. Saeidi, N. A. S. Amin, M. R. Rahimpour, J. CO2 Util. 2014, 5, 66–81.
49] P. Kangvansura, L. M. Chew, W. Saengsui, P. Santawaja, Y. Poo-arporn, M. Muhler, H. Schulz,
A. Worayingyong, Catal. Today 2016, 275, 59–65.
[
[
50] L. M. Chew, P. Kangvansura, H. Ruland, H. J. Schulte, C. Somsen, W. Xia, G. Eggeler, A.
Worayingyong, M. Muhler, Appl. Catal. A Gen. 2014, 482, 163–170.
51] T. Herranz, S. Rojas, F. J. Pérez-Alonso, M. Ojeda, P. Terreros, J. L. G. Fierro, Appl. Catal. A
Gen. 2006, 311, 66–75.
[
[
52] D. Mignard, C. Pritchard, Chem. Eng. Res. Des. 2006, 84, 828–836.
53] T. Inui, T. Yamamoto, M. Inoue, H. Hara, T. Takeguchi, J.-B. Kim, Appl. Catal. A Gen. 1999, 186,
3
95–406.
[
[
54] T. Inui, T. Yamamoto, Catal. Today 1998, 45, 209–214.
55] V. Subramani, S. K. Gangwal, Energy & Fuels 2008, 22, 814–839.
1
9
This article is protected by copyright. All rights reserved.

ChemSusChem
10.1002/cssc.201902340
[
[
[
56] G. Prieto, ChemSusChem 2017, 10, 1056–1070.
57] G. Tian, H. Yuan, Y. Mu, C. He, S. Feng, Org. Lett. 2007, 9, 2019–21.
58] G. Tian, C. He, Y. Chen, H.-M. Yuan, Z.-W. Liu, Z. Shi, S.-H. Feng, ChemSusChem 2010, 3, 323–
4
.
[
[
[
[
[
59] H. Ando, Y. Matsumura, Y. Souma, J. Mol. Catal. A Chem. 2000, 154, 23–29.
60] F. Bauer, H. G. Karge, Mol. Sieves - Sci. Technol. 2006, 5, 249–364.
61] W. O. Haag, R. M. Lago, P. G. Rodewald, J. Mol. Catal. 1982, 17, 161–169.
62] J. F. Haw, W. Song, D. M. Marcus, J. B. Nicholas, Acc. Chem. Res. 2003, 36, 317–326.
63] M. Bjørgen, S. Svelle, F. Joensen, J. Nerlov, S. Kolboe, F. Bonino, L. Palumbo, S. Bordiga, U.
Olsbye, J. Catal. 2007, 249, 195–207.
[
[
64] F. Yin, M. R. Li, G. C. Wang, Phys. Chem. Chem. Phys. 2017, 19, 22243–22255.
65] H. Ma, Y. Chen, Z. Wei, S. Wang, Z. Qin, M. Dong, J. Li, J. Wang, W. Fan, ChemPhysChem 2018,
1
9, 496–503.
[
[
[
66] S. Ilias, A. Bhan, ACS Catal. 2013, 3, 18–31.
67] T. Wang, Y. Xu, C. Shi, F. Jiang, B. Liu, X. Liu, Catal. Sci. Technol. 2019, 9, 3933–3946.
68] L. Quintana Gomez, HYDROTHERMAL CONVERSION OF CO2 INTO HIGHER HYDROCARBONS
AND, University of Sheffield, 2017.
[
[
[
[
69] S. Liu, Y.-W. Li, J. Wang, H. Jiao, J. Phys. Chem. C 2015, 119, 28377–28388.
70] H.-J. Li, J.-J. Ho, J. Phys. Chem. C 2010, 114, 1194–1200.
71] S. M. Csicsery, Zeolites 1984, 4, 202–213.
72] M. Andérez-Fernández, E. Pérez, A. Martín, M. D. Bermejo, J. Supercrit. Fluids 2018, 133, 658–
6
64.
2
0
This article is protected by copyright. All rights reserved.